1. Field of the Invention
This invention relates generally to organic light emitting diodes, and, more particularly, to electrophosphorescent organic light emitting diodes.
2. Description of the Related Art
An organic light-emitting diode (OLED) is a thin-film light-emitting diode that uses an organic compound as an emissive layer. FIG. 1 conceptually illustrates a conventional OLED 100 that includes an emissive layer 105 sandwiched between an anode 110 and a cathode 115. The anode 110 is typically formed on a glass substrate 117 using a transparent material such as indium tin oxide (ITO). Holes 120 may be provided to the emissive layer 105 via the anode 110. The cathode 115 is typically formed of a metal (such as aluminum or calcium) and is used to provide electrons 125 to the emissive layer 105. In some embodiments, the cathode 115 may also be transparent. The holes 120 and the electrons 125 in the emissive layer 105 may combine to form excitons 130. The excitons 130 decay when the hole 120 and the electron 125 combine and release the energy stored in the exciton 130 as heat and/or light.
A portion of the light released when the exciton 130 decays is emitted through the anode 110 and the glass substrate 117, as indicated by the arrow 135. However, some of the light released when the exciton 130 decays (indicated by the arrow 140) intersects the interface 145 between the glass substrate 117 and the air at an angle 150 relative to a normal 155 to the interface 145 that is larger than a critical angle for total internal reflection. Consequently, the light 140 is reflected back into the OLED 100. Over 80% of the light generated in a conventional planar OLED 100 may be lost to internal reflection and waveguiding. Light released by the exciton 130 may also be totally internally reflected at other interfaces, such as the interfaces between the emissive layer 105 and the anode 110 and/or the interface between the anode 110 and glass substrate 117. However, the contrast in indices of refraction is typically largest at the interface 145, so total internal reflection at the interface 145 is typically the largest contributor to the reduction in the external quantum efficiency of the OLED 100.
One well-known technique for increasing the external quantum efficiency of the OLED 100 is to form a scattering (or relief) structure at or near the interface 145. A wide range of integrated optical devices and optoelectronic devices include relief structures with features sizes comparable to the wavelength of the light emitted by the emissive layer 105. The relief structures may function as mode and/or output couplers, filters, laser resonators, and other components. Fabrication of the relief structures is typically accomplished with modified versions of techniques—photolithography and electron beam lithography—that have their origins in the microelectronics industry. Integrating similar elements into OLEDs 100 may improve their external quantum efficiencies by coupling photons out of planar waveguides defined by the various device layers. However, these traditional optoelectronic fabrication techniques are poorly suited to use in OLEDs 100, at least in part because of the operational complexities of the techniques, high costs, and/or the inability to pattern large areas. Furthermore, only a narrow range of OLED materials are chemically compatible with the resists, solvents and developers used by these methods.